In a usual setup, an indirect electrical converter comprises an input rectifier, which from an AC input voltage generates a DC link voltage. The DC link voltage is supplied to an output inverter, which generates an AC output voltage, which may be used for driving an electrical machine. Contrary to this motoring mode, it is also possible to use the electrical machine as generator and to convert the AC voltage with variable frequency into an AC voltage with constant frequency to be supplied to an electrical grid.
To allow a better voltage utilization of a multi-phase converter (input rectifier and output inverter), when operated with ungrounded neutral point or star-point, typically overmodulation is used. During overmodulation, a common mode voltage may be added to the fundamental phase voltages, with a dominant component at three times the frequency of the fundamental phase frequency.
However, in this case, the common mode voltages added to the phase voltages on the input rectifier and output inverter may stress the insulation of the converter system and the insulation of connected AC components, i.e of the electrical machine and of the transformer. Therefore, the maximal torque generated with a common mode voltage is limited by the insulation capabilities of the converter system and the connected AC components.